This invention relates to polyphenylene ether-polyamide blends, and more particularly to the preparation of such blends without the use of compatibilizing compounds.
Blends of polyphenylene ethers and polyamides, in which the polyamide is generally the continuous phase, are in wide commercial use in applications requiring a combination of such properties as temperature stability, impact resistance and solvent resistance, as illustrated by the fabrication of such articles as exterior body parts for automobiles. It has long been known, however, that such blends are difficult to prepare because of the incompatibility of polyphenylene ethers with polyamides. As a result of this incompatibility, such blends typically undergo phase separation and delamination and contain large, incompletely dispersed polyphenylene ether particles and no phase interaction between the two resin phases. Molded parts made from such blends are, as a result, usually characterized by extremely low impact strength, brittleness, delamination and the like.
One strategy which has been employed to improve the properties of polyphenylene ether-polyamide blends is to introduce compatibilizing compounds which facilitate copolymer formation between the two resins. Typical compatibilizing compounds contain such functional groups as olefinic, carboxylic acid, ortho ester, epoxide and chlorotriazine groups; illustrative compounds of this type are maleic anhydride, fumaric acid, trimellitic anhydride acid chloride and 2-chloro-4-(2,4,6-trimethylphenoxy)-6-glycidoxy-1,3,5-triazine. The use of such compounds is disclosed in many U.S. patents, including U.S. Pat. Nos. 4,315,086, 4,600,741, 4,732,937, 4,873,286, 5,100,961, 5,089,567 and 5,115,042.
In a typical blending method which includes the use of compatibilizing compounds, a twin screw extruder configured to provide severe mixing intensity is employed. The feed throat of the extruder is charged with polyphenylene ether, compatibilizing compound, a suitable impact modifier as detailed hereinafter, any required stabilizers and a portion of the polyamide. The polyamide is present to protect the impact modifier, often an unsaturated rubbery polymer, from degradation caused by friction-generated heat in the upstream portion of the extruder.
Extrusion in the upstream region is at a temperature high enough to cause reaction of the compatibilizing compound with the polyphenylene ether, said temperature typically being up to about 290.degree. C. Atmospheric venting of the extruder is employed upstream. The remainder of the polyamide is introduced downstream with vacuum venting, at a temperature typically in the range of about 290-300.degree. C.
The use of such compatibilizing compounds and conditions is, however, often accompanied by disadvantages. For example, the compatibilizing compound can also react to link polyamide molecules, causing chain lengthening or crosslinking which may impair the flow properties of the blend. Depending on their chemical nature, they can further cause corrosion of blending equipment such as extruder screws and shafts and present environmental hazards. It goes without saying that they also add to the material costs of the blending process.
The preparation of uncompatibilized polyphenylene ether-polyamide blends is disclosed in U.S. Pat. No. 3,379,792. Because of the incompatibility of the resin phases, however, such blends are disclosed as having inferior physical properties if the polyamide is present in greater amount than 20% by weight. In such blends, the polyamide serves only as a flow modifier for the polyphenylene ether. Clearly, there is no opportunity for the polyamide to constitute the continuous phase or to contribute significantly to the properties of the composition.
It is desirable, therefore, to develop new methods for compatibilization of polyphenylene ether-polyamide blends which do not require the use of compatibilizing compounds which degrade blend properties. It is especially desirable to prepare compatible blends in which the polyamide is the continuous phase.